Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic system, an electrostatic recording system, or other systems.
Description of the Related Art
A color copying machine including image forming units for a plurality of colors generally has a color image forming mode in which all of the image forming units are activated to form a color image, and a monochrome image forming mode in which only the image forming unit for black is activated to form a monochrome image. Further, when the monochrome image is to be formed, the image forming units for colors other than black are not required to be set in an image formable state. Therefore, the image forming modes are switchable depending on contents of a job input to the copying machine. The “job” herein refers to information for executing a series of image forming operations onto a single sheet or a plurality of sheets based on an instruction to start the image forming operations.
For example, in an image forming apparatus disclosed in U.S. Pat. No. 7,420,583, there is disclosed a technology of reducing a time period required for switching from the monochrome image forming mode to the color image forming mode.
In this image forming apparatus, an operation of preparing for image formation in the color image forming mode is started during the image formation in the monochrome image forming mode, and the image formation in the color image forming mode is started after the image formation in the monochrome image forming mode is ended.
FIG. 6 is a timing chart for illustrating the image forming mode switching operation of the image forming apparatus disclosed in U.S. Pat. No. 7,420,583. In FIG. 6, Y represents a yellow color, M represents a magenta color, C represents a cyan color, and K represents a black color. Further, “IMAGE FORMATION K1” represents the image formation in the monochrome image forming mode, and “IMAGE FORMATION Y2”, “IMAGE FORMATION M2”, image formation C2, and “IMAGE FORMATION K2” represent the image formation in the color image forming mode. “PREP” represents the operation of preparing for the image formation.
In the image forming apparatus disclosed in U.S. Pat. No. 7,420,583, from an end time ti of the image formation by the image forming unit for black during the monochrome image forming mode, the image forming unit located at the most upstream position in the color image forming mode (in this case, the image forming unit for yellow) starts its image formation. Therefore, the image forming unit for black does not form an image during a period Tw, and there arises a problem in that a downtime (out of operation time) occurs due to the switching of the image forming mode.
The present invention has a primary object to provide an image forming apparatus capable of reducing a downtime due to switching of an image forming mode.